Two of the most important operating factors entering into the
consideration of what constitutes a satisfactory boiler are its
efficiency and capacity. The relation of these factors to one another
will be considered later under the selection of boilers with reference
to the work they are to accomplish. The present chapter deals with the
efficiency and capacity only with a view to making clear exactly what is
meant by these terms as applied to steam generating apparatus, together
with the methods of determining these factors by tests.
Efficiency--The term "efficiency", specifically applied to a steam
boiler, is the ratio of heat absorbed by the boiler in the generation of
steam to the total amount of heat available in the medium utilized in
securing such generation. When this medium is a solid fuel, such as
coal, it is impossible to secure the complete combustion of the total
amount fed to the boiler. A portion is bound to drop through the grates
where it becomes mixed with the ash and, remaining unburned, produces no
heat. Obviously, it is unfair to charge the boiler with the failure to
absorb the portion of available heat in the fuel that is wasted in this
way. On the other hand, the boiler user must pay for such waste and is
justified in charging it against the combined boiler and furnace. Due to
this fact, the efficiency of a boiler, as ordinarily stated, is in
reality the combined efficiency of the boiler, furnace and grate, and
Efficiency of boiler,} Heat absorbed per pound of fuel
furnace and grate } = ------------------------------- (31)
Heat value per pound of fuel
The efficiency will be the same whether based on dry fuel or on fuel as
fired, including its content of moisture. For example: If the coal
contained 3 per cent of moisture, the efficiency would be
Heat absorbed per pound of dry coal × 0.97
------------------------------------------
Heat value per pound of dry coal × 0.97
where 0.97 cancels and the formula becomes (31).
The heat supplied to the boiler is due to the combustible portion of
fuel which is actually burned, irrespective of what proportion of the
total combustible fired may be.[54] This fact has led to the use of a
second efficiency basis on combustible and which is called the
efficiency of boiler and furnace[55], namely,